I answered this question on Quora on 6-22-16 –Barry Edwards
It completely depends on what type of business that you own. Is it B2B (Business to Business) or B2C (Business to Consumer)?
For the business to consumer start up
I’ll just summarize as the question is too broad, but if it’s B2C, like an e-commerce website, and you have no audience, then your best bet is to find your audience on social media and popular blogs and get involved in conversations. People will become interested in you and seek more info. During this time, you should be building really good content on your blog that is appealing to your target audience, so that they’ll become engaged (lower bounce rate), bookmark your page, share your product or content on social media… maybe even purchase!
Credibility is everything. Get people to rate you on Yelp, Google or whatever channels work best for your industry as soon as possible.
For the business to business start up
If your business is B2B, and you have no audience, I’d suggest starting with networking groups/chambers of commerce and collect email addresses. Upon establishing rapport say, “Hey, I’ve got a small, local email group. We like to share like ideas. Can I add you to the list?” They’ll almost always say yes. *Make sure you send them 80% non-spam, quality, content of value*. Basically, you should be building your landing pages with this same content and nurturing your small, but building email list. I’d also add that when trust is established with these people, reach out to them and ask them what their biggest challenges are that happen to be in your area of expertise – this is your subject matter for future website/email content.
Credibility is everything. Create content: video, podcasts, eBook… whatever works for you and offer this on your home page and social media channels.
How do I know how much to invest in my online marketing?
You need to assess how valuable a lead is to you. For instance, a dentist may look at obtaining a new, regular patient as being worth an average of $600/year. While a plastic surgeon (a main client of ours) may value a new patient at an average of $10k. Since it takes 3 hot leads (they seek you out) to make 1 conversion, I consider the plastic surgeons leads to be worth $3300 ea. Needless to say, we put a lot of effort into this site on a monthly basis to ensure the leads keep coming in (continuous SEO optimization, content marketing and website maintenance).
The dentist should only do local-optimized SEO, on-page SEO, and what content marketing efforts he/she can integrate in house with a trained staff member. A well designed and implemented marketing campaign for this dentist can yield the same ROI as the plastic surgeon if a properly scaled.
I hope that at least somewhat answers your question. There are nearly limitless ways to market and the best channels for you depends largely on your industry and target audience.